The present invention relates to photovoltaic cells, also known as solar cells, for producing electricity from sunlight, and more particularly, to broad footprint photovoltaic cells utilizing electrolytes containing iodine-based redox species or other corrosive species.
The invention has particular relevance for solar cells of the dye-sensitized type, and is applicable to other types of solar cells in which a high current density of operation at minimal ohmic loss is advantageous.
Dye-sensitized photovoltaic cells for producing electricity from sunlight have been disclosed by U.S. Pat. No. 5,350,644 to Graetzel, et al., which is hereby incorporated by reference for all purposes as if fully set forth herein. U.S. Pat. No. 5,350,644 teaches a photovoltaic cell having a light-transmitting, electrically-conductive layer deposited on a glass plate or a transparent polymer sheet to which a series of titanium dioxide layers have been applied, in which at least the last titanium dioxide layer is doped with a metal ion that is selected from a divalent or trivalent metal.
Following U.S. Pat. No. 5,350,644 is U.S. Pat. No. 6,069,313 to Kay, which is hereby incorporated by reference for all purposes as if fully set forth herein. U.S. Pat. No. 6,069,313 teaches a plurality of series-connected cell elements arranged as separate, parallel, narrow elongated strips on a common transparent substrate. Each element includes a light facing anode including nanocrystalline titania, a carbon-based counterelectrode (cathode), and an intermediate electrically insulating porous layer separating the anode from the cathode. The pores of the intermediate layer are at least partially filled with a liquid phase, ion-transferring electrolyte, following coating of the nanocrystalline titania with a light sensitive dye.
A current collecting layer of a tin oxide based transparent, electrically-conducting material is situated between the transparent substrate and the anode. The anode and cathode of a given cell provide a direct-current voltage when the anode is exposed to light, such that series assemblies of cells may readily be built up. The cathode of each succeeding element is connected with the intermediate conducting layer of the preceding anode element, over a gap separating the respective intermediate layers of these two elements. The series of cells is then sealed using an organic polymer, ensuring in particular that each individual strip cell is sealed from its neighbor cell. In the art of dye cells, such an arrangement of anode and cathode materials or cells on a common substrate is termed “monolithic”.
Generally, dye cells of the above-cited prior art disclosures are much closer conceptually to battery cells than to conventional photovoltaic cells, since the charge generators are separated by an electrolyte and are not in direct contact. These cells have two electrodes separated by an electrolyte, with one electrode (the photo-electrode or photo-anode) facing the sun or light source. Each electrode is supported on its own current collector, usually a sheet of conducting glass, which is glass coated on one side with a thin (˜0.5 micron) transparent layer, usually based on electrically-conductive tin oxide. The conducting glass sheets act as transparent walls of the dye cell.
A transparent polymer may be used in place of glass to support the tin oxide. The photo-electrode or photoanode includes a transparent porous layer about 10 microns thick (in contact with the tin oxide layer) based on titania, having a nanocrystalline characteristic particle size of 10-50 nm, applied by baking onto the conductive glass or transparent polymer, and impregnated with a special dye. The baked-on titania layer is applied in dispersion form by any of various methods: doctor-blading, rolling, spraying, painting, electrophoresis, gravure printing, slit coating, screen printing or printing. The baking step giving highest cell performance is usually in excess of 450° C., requiring the use of conducting glass rather than plastic for supporting the titania layer. Other processing procedures for the titania layer are feasible, such as reduced temperature baking, or pressing, usually with some sacrifice in efficiency. It is important to note that the titania is principally in contact with the tin oxide. Presence of other conductors (such as many metals, carbon and the like, even if chemically inert to the electrolyte) on the photoanode can greatly increase recombination of charge carriers and provide a serious efficiency loss in the cell. Very few materials (amongst them tin oxide and titanium metal) combine chemical inertness to the electrolyte with freedom from recombination effects so as to be potentially suitable as construction materials for, or for electrically associating with, the photoanode.
For cells that are partially transparent, the other electrode (the counter electrode) includes a thin layer of catalyst (usually containing a few micrograms of platinum per sq. cm) on its respective sheet of tin oxide coated conductive glass or transparent plastic. If cell transparency is not required, the counter electrode can be opaque, for example, based on carbon or graphite advantageously catalyzed with trace platinum or another catalyst. The electrolyte in the cell is usually an organic solvent with a dissolved redox species. The electrolyte is typically acetonitrile or a higher molecular weight reduced volatility nitrile, with the redox species being dissolved iodine and potassium iodide—essentially potassium tri-iodide. Other solvents and phases, for example ionic liquids with no vapor pressure, and even different redox species, may be used, however.
U.S. Pat. No. 5,350,644 to Graetzel, et al., discloses various dye cell chemistries, especially different dyes based on ruthenium complexes. Photons falling on the photoelectrode excite the dye (creating activated oxidized dye molecules), causing electrons to enter the conduction band of the titania and to flow (via an outer circuit having a load) to the counter-electrode. There, the electrons reduce tri-iodide to iodide in the electrolyte, and the iodide is oxidized by the activated dye at the photoanode back to tri-iodide, leaving behind a deactivated dye molecule ready for the next photon. U.S. Pat. No. 5,350,644 discloses that such dye cells can attain a solar-to-electric conversion efficiency of 10%.
The cells taught by U.S. Pat. No. 5,350,644 to Graetzel, et al., are based on two generally parallel, closely placed sheets of conductive glass, a first sheet coated with titania and dye, and a second sheet coated with catalyst. Electrolyte is provided between the two sheets. The cells are sealed by means of organic adhesive at the edges. Current takeoff is achieved via the conductive glass sheets, which project beyond the adhesive on either side of the cell. These cells operate at a voltage of about 700 mV and at a current density of 15 mA/sq. cm under peak solar illumination, with the counterelectrode being the positive pole. It is asserted therein that since the materials and preparation methods are low cost and the titania layer can be prepared in large areas, such cells could potentially provide a good route to low-cost photovoltaic cells. It is further argued that there might be significant cost savings over classical single crystal or polycrystalline silicon cells and even more recent thin-film photovoltaic cells, since these are all high cost and rely on expensive and often environmentally problematic raw materials, together with complex, costly, semiconductor industry processing equipment and production techniques. These drawbacks include the use of vacuum deposition and semiconductor doping methods, clean-room protocols, use of toxic hydrides such as silane, phosphine, etc., as raw materials, and the use of toxic active-layer materials containing cadmium, selenium or tellurium.
The ohmic loss via the conductive glass coated with tin oxide is a major problem of dye cells. The tin oxide coating is extremely thin, being limited in thickness usually to below one micron due to the need to maintain a high light transmittance through to the dye/titania layer of the photoanode. Moreover, tin oxide is only semiconductive and is bonded to with difficulty. Consequently, the current takeoff is significantly limited by such a cell design to very small sized cells having a maximum footprint of a few square millimeters or strip cells having long narrow strips of active titania. It should be noted that active strip cells have certain technical disadvantages. For example, in the cells of Kay described above, the strips of titania are disadvantageously narrow (typically 0.6-0.8 cm wide), due to the ohmic loss restriction. This results in an excessive loss of active area between cells, due to the practical width of inert materials needed for inter-cell sealing. Moreover, adequate sealing between adjacent cells so as to effectively prevent any inter-cell electrolyte migration remains a serious challenge.
Efforts have been made to increase the active area and breadth of cells by laying down parallel conducting strips on a conducting glass surface, thereby enabling a large-area, broad-cell construction. A photovoltaic cell having electrically conducting coatings on spaced, glass support panes is disclosed by U.S. Pat. No. 6,462,266 to Kurth, which is hereby incorporated by reference for all purposes as if fully set forth herein. In FIG. 1 is provided a partial, cross-sectional view of a photovoltaic cell 1 having two generally parallel support panes 2 and 3, set apart at a distance. The edges of photovoltaic cell 1 are held and sealed by a sealing system 4 extending along the whole circumference. The inner surfaces of support panes 2 and 3 are each coated with a conductive layer 5, and 6, respectively. Layers 5 and 6 are made of a suitable metal or metal oxide such as tin oxide. On layers 5 and 6, an arrangement of parallel conductor leads 7 and 8 are provided, preferentially made from silver or a silver alloy, or from copper or a copper alloy. These conductor leads are coated each with an insulating coating 10, which insulates conductor leads 7 and 8 electrically towards the interior of the cell. Coating 10 consists of a glass free of heavy metals, which was applied as a glass flow on conductor leads 7 and 8. Onto conductor leads 7 and 8 insulated by the glass coating 10, a further electrically conductive layer 11, and 12 respectively, made from tin oxide or a similar material, can be applied in order to obtain a still-higher yield of photovoltaic cell 1. In the border zones of seals 13 and 14, no electrically conductive layers 5 and 6 are provided, these layers having been eliminated from the border zones by means of sand blasting. In this manner, possible short circuits via the seals 13 and/or 14 are avoided. Onto these two seals, a thin layer 15 of a low melt soldering tin is applied in such a manner that exterior weather influences also can not act onto photovoltaic cell 1.
The photovoltaic cell taught by U.S. Pat. No. 6,462,266 has reduced ohmic loss with respect to the cell disclosed by U.S. Pat. No. 5,350,644 to Graetzel, et al., because conductor leads 7 and 8 are good conductors (e.g., silver paste screen-printed on and fired at 600° C.), and because the overall effective thickness of conducting materials has been increased. Unfortunately, the high process temperatures required for the silver and glaze compositions adversely affect the conductivity and strength properties of the glass. Glaze materials that can be processed at lower temperatures are available, but contain toxic heavy metals such as lead, and may also be attacked or contaminated by the electrolyte. Furthermore, protection of silver conductors by a glass layer is inadequate over time, since pinholes cannot be entirely eliminated in the glaze, especially in mass production processes. In fact, silver is corroded by the iodine in cell electrolytes, adversely reducing the iodine inventory in the cell and irreversibly attacking the dye, and a similar deterioration occurs with silver lines protected by polymer.
U.S. Patent Application Publication No. 20050072458 to Goldstein describes a large-area, broad conductive glass or conductive plastic for a dye cell. The conductive glass or conductive plastic carries a set of conductors selected from materials intrinsically resistant to corrosion and to carrier recombination in the presence of the cell electrolyte, and onto this superior glass or plastic (having enhanced current collecting properties over plain conductive glass or plastic) the titania is deposited. By way of example, a conductive glass face is first grooved, giving a set of parallel spaced shallow grooves. Into each groove is placed a wire of a metal such as titanium, molybdenum, tungsten, chromium or their alloys (inert to corrosion and to carrier recombination under the operating conditions of the cell) and electrical conductivity between the wire and the tin oxide layer on each side of the groove is achieved using a heat curable binder paste based on an inert ceramic adhesive (such as alumina) mixed with an inert, electrically conducting filler (such as titanium nitride). The paste fills the groove and overlaps on each side of the groove to make good electrical contact with the tin oxide layer there following curing. The wires exit from the cell from the groove extremities at the glass edges and may be welded to a current-collecting strip. In a separate embodiment of U.S. Patent Application Publication No. 20050072458 to Goldstein there is described a set of parallel strips of a metal or metal alloy having stability under cell operating conditions that is plated onto the conductive glass or conducting plastic surface. One example given of a plated metal is chromium. Current take-off from the anode plate is again made from the side of the cell.
In another embodiment, the parallel conductors are inert strips or wires of titanium, molybdenum, tungsten, chromium or their alloys bonded directly to the conducting surface of the glass by means of an inert, electrically conducting ceramic adhesive.
U.S. Patent Application Publication No. 20050072458 extends the use of wires in a grooved conductive glass or of strips plated on conductive glass also for use in the counterelectrode (cathode) of the cell. The glass plate, provided with wires bonded in grooves or on top of the glass, or with strips electroplated on the conductive surface, is used as a base for a broad cathode in the dye cell, and the conductivity-augmented plate is covered with a catalytic layer electroactive to iodine. Such a cathode, unfortunately, although fitted with adequate conducting means for current takeoff from a large area broad cell, necessarily includes a second layer of conducting glass in the cell, with associated cost, weight and thickness penalties. Broad dye cells of at least 10-15 cm per side are made possible, however.
To date, there has been no real commercialization of photovoltaic dye cells, despite the great techno-economic potential thereof. The principal problems remaining include scale-up of cells to widths much above one centimeter—and areas much above 50 sq. cm—due to excessive ohmic losses from the poorly conducting tin oxide layers on the glass or plastic, long term stability of silver based conductors in the cell, and difficulties of sealing the cells against long term dryout and performance degradation. Many approaches rely on costly in-house coating of the transparent substrate with conductive tin oxide rather than using commercially-available conductive glass in bulk such as fluorine doped tin oxide (FTO) glass.
A further problem in prior art dye cells and modules has been excessive surface area wasted in seals, protective layers and conducting paths on the sun-facing side of the cell or module. The active current-producing area in such cases is often less than 70% of the geometric area (footprint) of the cell or module, providing a poor effective efficiency from the available area. A yet further problem in prior art cells, especially if the counterelectrode is a separate element from the anode, is the relatively large spacing between anode and cathode—approaching or exceeding 100 microns—which can result in excessive ohmic loss from the electrolyte. This problem is particularly acute in the dye cells disclosed by U.S. Patent Application Publication No. 20050072458 to Goldstein, and is even more particularly acute in the disclosed dye cells having a large footprint or cell width.
Moreover, it should be evident from the above that when the counterelectrode is a separate element from the anode in the cell, the counterelectrode usually requires the use of a second glass support in the cell, or even worse, a second conductive glass in the cell. These greatly add to the cost, weight and thickness of the cell.
There is therefore a recognized need for, and it would be highly advantageous to have, an electrochemical cell, powered by sunlight, that is simple, large-area, broad, efficient, low-cost, lightweight and robust, and successfully addresses the shortcomings of the prior art.